Many devices for processing optical beams have significant size restrictions, yet require relatively long lengths for the beams inside the devices.
A common technique for increasing the length of a beam within an optical device is to fold the beam inside the device. Binoculars, for example, commonly reflect incident optical beams in a “Z” shape to increase their optical length while keeping their physical length small. Many reflex cameras employ a pentaprism to increase the optical length of their viewfinders within a small camera body. Projection systems of various kinds attain wider deflection angles in a shorter distance from projector to screen by folding their beams internally. Projection television receivers, for example, frequently employ shaped mirrors to fold the beams traveling from the guns to the screen. Collimators and other types of devices can achieve increased beam length by folding incident optical beams within the devices. Heads-up displays and helmet-mounted military optics require multiple optical functions in a small volume. Other kinds of radiation beams, such as x-rays and electron beams, can also be processed by folding them in suitable devices.
A number of conventional devices fold optical beams with plane mirrors or other reflectors. Beam folders implemented with conventional reflectors generally do not save large amounts of space. That is, at least one physical dimension of such a device remains a large fraction of the effective optical length within the boundaries of the device. Other conventional folding devices employ beam splitters. These devices can significantly increase optical length compared to their physical dimensions. However, beam splitters typically suffer from low optical efficiency. The intensity of the output beam is frequently only a small fraction of the incident beam's strength. In adsdition, such devices do not perform other optical functions within the device. Their only function is to increase path length; any further beam manipulation must be accomplished separately, thus increasing the overall volume of the device.